Passive elements are used in all manner of electronic circuits. A wide variety of implementations are known for implementing passive circuits, including lumped element and distributed element designs. In certain applications microstrip and stripline designs have also been employed. For example, a stripline is a printed signal path disposed between two ground planes in a printed circuit board. Many single-element stripline circuits are known and used in the industry (e.g., as implemented by the Anaren Xinger couplers), and disclosures have been made about extending a single element stripline into two layers (see, e.g., U.S. Pat. No. 5,929,729 to Swarup (where coupling was an objective), and U.S. Pat. No. 5,359,304 to Fujiki.
In certain high frequency applications, prior approaches towards designing passive elements have proved unsatisfactory. For example, lumped-element harmonic filters tend to result in poor manufacturing yield, and board-to-board performance variation is typical, requiring expensive hand-tuning. On the other hand, distributed element designs are not typically used where there is limited board space and the possibility of re-entrant bands (e.g., passbands repeated at harmonic frequency, to suppress out-of-band rejection). If the application has to be low loss, the design constraints become even more difficult. For example, where the radio-frequency (RF) is above 1 GHz, many applications will require limiting the radiation of unintended signals (spurs, harmonics, etc) to maximize frequency spectrum reutilization. Such requirements drive the need for passive circuits like low pass filters that provide the lowest possible insertion loss and maximum out-of-band rejection. The conventional lumped-element elliptic low pass filters are very difficult to implement at frequencies above 1 GHz due to small component values and extreme sensitivity to component tolerance. Stripline circuits would not be considered by a typical designer, since the physical dimensions of distributed elements at frequencies below 2 GHz are prohibitively large. This rules out the use of distributed element filters when more than one filter has to be implemented in a constrained board space.
There remains, therefore, a need for a better approach to circuit design for multiple high frequency (i.e., greater than 1 GHz), low loss passive elements for use in constrained-size applications. Just such an approach is now possible by the invention described in more detail in connection with the following embodiment.